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Fear and the brain.

Posted Sep 22 2008 11:07am

Imagine you’re alone in the business district of a big city. The day is over; the sun has set. The people who work here are all home by now, safe and warm, reading or having conversations or watching a bit of television before turning in for the night. Your footsteps echo back and forth between the office buildings lining the street.

At the corner, you stop to wait for the light. Suddenly, you notice a large man emerging from the darkness of a nearby doorway while reaching to pull something from his jacket pocket. Something metallic, that glints in the light from the streetlamps.

Instantly, you freeze in your tracks. Everything else drops out of your consciousness – your thoughts about a conversation you had earlier in the day, your plans for the weekend – and you study the man emerging from the doorway out of the corner of your eye. Is that a knife he’s holding? Or a gun? Meanwhile, automatically, your breathing quickens and your muscles tense. You’ve sensed danger, which has triggered the fight-or-flight response.

This response exists in all vertebrates, and a variety of other animals as well. And for good reason, from an evolutionary standpoint.

Picture a field mouse, for instance. When it first notices the outline of what might be a bird of prey high overhead, it freezes, making it far less likely to become the predator’s next meal, then decides whether the threat to its well-being is real, and if so, what to do next – flee for safety, or turn and fight. A mouse that doesn’t exhibit the fight-or-flight response is a mouse that’s unlikely to stick around to pass on its genes.

Thanks to recent neurological research, we now know what happens in the brain to cause this automatic response. First, information from your senses is sent to the amygdala, a tiny, almond-shaped region deep in the center one of the more primitive areas the brain – and the linchpin of the brain’s fear circuitry – which begins conducting a quick-and-dirty assessment of the threat implicit in that information.
In the case of our thought experiment, the amygdala, an early-warning sentry of sorts, makes the snap decision that, yes, the figure emerging from the shadowy doorway may well pose you a threat. Almost immediately, it sounds a neurological “red alert” that activates your sympathetic nervous system, flooding your body with chemicals including the hormones epinephrine, norepinephrine, and cortisol. This is the jolt you experience when faced with a potential threat to your well-being, from the car that cuts you off unexpectedly to the homeless guy who starts yelling at you suddenly and for no reason.

The activation of the sympathetic nervous system causes a variety of physical responses. To make sure the big muscles of your legs, arms, and torso have the extra oxygen it needs to fight or flee, your breathing quickens, your heart rate spikes, and blood flow is redirected from your skin, hands, feet, kidneys, and gastrointestinal tract. You begin sweating to prevent your body from overheating. Your ability to concentrate is enhanced. To improve your vision, your pupils dilate. Your sense of pain dulls, your reflexes sharpen, and to prevent hemorrhaging should you be injured during the crisis, your blood becomes more readily coagulated. All this begins before you even realize what you’re reacting to – in the case of our thought experiment, before you’re consciously aware that you’ve seen a figure emerge from a shadowy doorway.

At the same time as your brain is sending sensory information to your amygdala, it’s also sending that information to your prefrontal cortex, the home of advanced cognitive functions like judging and planning, and a much more recent evolutionary development in the brain. The prefrontal cortex conducts a much more careful, rational analysis of the sensory information. If it decides that the amygdala was wrong, and there’s no rational cause for alarm, it issues a “stand down” order, telling the amygdala to quiet and triggering the parasympathetic nervous system to calm the excitement stirred up by the fight-or-flight response. But because of the way the brain is wired – because information from your senses is transmitted to your prefrontal cortex much more slowly than to the amygdala – this takes place only after the amygdala has sounded the red alert.

*****

Back to our thought experiment: What if, after the adrenaline starts pumping through your system, you realize that the man emerging from the shadowy doorway is not in fact a threat? What if you see that the scary-looking metallic thing he’s got in his hand is in fact a cellphone? What if you watch him flip the phone, hear him begin to speak: “Right, a dozen eggs and a half-gallon of two-percent milk. Don’t worry, dear, I’ll stop by the grocer’s on the way home”?

For most people, the result is a sigh of relief. Thanks to the red alert sent out by the amygdala, you’ll still feel the adrenaline coursing through your system for a minute or two. But because your prefrontal cortex has analyzed the situation and concluded that there’s no real danger to you, you’ll stop feeling fear. Within a couple of blocks, you may have forgotten the episode altogether.

Unfortunately, people with panic disorder display less prefrontal-cortex activity of the kind needed to switch off the amygdala than do others. In other words, the panic sufferer’s stand-down signal is a mere whisper, when what’s needed is a signal more along the lines of a shout. The result? The amygdala continues to sound the red alert, making it exceedingly difficult to exit the fight-or-flight response. And a situation like an evening encounter with a guy on a cell phone can become confrontations with the most base, primal kind of fear.
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